Published in conjunction with Matthew Hollis's fine biography of the poet, this selection contains 80 of the 140-odd poems that Edward Thomas produced in the two years that he devoted to writing verse. It's a generous selection from a small body of work. But of more interest to those already familiar with the poet whom Ted Hughes characterised as "the father of us all" are the extracts from diaries, notebooks and articles that preface the book.

In 1913, aged 35, Thomas had spent half a career as a tireless reviewer of others' poetry and a writer of more or less well-regarded prose books about the English and Welsh countryside, literary biographies and essays. According to one estimate, he produced well over a million words in reviews alone. Given that Thomas aspired to write poetry himself, it's hardly surprising that he viewed his work as frustrating drudgery, and that the need to keep churning it out exacerbated the depression to which he was prone. It took a poet – Robert Frost – to tell him that he "was writing as good poetry as anybody alive, but in prose form where it did not declare itself". The prose extracts Hollis includes are well chosen to illuminate the revolutionary shift that Frost enabled Thomas to make, as well as outlining significant aspects of his ars poetica.

It's fascinating to follow the evolution of the first poem Thomas wrote. A brief diary entry for 2 November 1914 led to a fuller prose piece dated two weeks later, titled "The White Horse" . This records his first visit to the pub of that name in what was then a remote and inaccessible corner of Hampshire, up on the Froxfield plateau, and the words spoken to him by the woman who fetched his beer: "I shd like to wring the old girl's neck for coming away here." Two weeks more, and he'd written "Up in the Wind", a blank verse dialogue that begins: "'I could wring the old thing's neck that put it here! / A public-house! it may be public for birds, / Squirrels and such-like, ghosts of charcoal-burners / And highwaymen.' The wild girl laughed . . ."

If that poem owed much in terms of both tone and subject matter to Frost's recent work – in particular, North of Boston, which Thomas had reviewed rapturously the summer before – it took him hardly any time at all to find his own distinctive voice. Another diary entry from late 1914 records Thomas's younger daughter, Myfanwy, walking down the path to their house in Steep, and plucking leaves from the artemisia that grew by the door. This in turn forms the basis for "Old Man", written that December. The poem's succession of ideas – the plant, the child, the poet's memory – follows that of the prose piece exactly; but where the latter is a concrete and descriptive piece of observation, the poem is questioningly atmospheric. There's a subtle, carefully handled shift in cadence from the one to the other. The diary ends with the writer recalling "only a dark avenue without end". This is transmuted to the poem's quietly terrifying final line: "Only an avenue, dark, nameless, without end." Thomas's simple summation of Frost's work is applicable to him too: "It is poetry, because it is better than prose."

One can always argue with the choices an editor makes – I'm sorry there's no room here for two of my own favourites, "Sowing" and "Women he liked" – and it's not clear why Hollis follows a different order from that of the two volumes, Poems and Last Poems, that Thomas didn't live to see printed. But these are quibbles. For those who don't know Thomas, this is an excellent introduction to the work of arguably the most influential English poet of the 20th century; and for those who already love him, the perspectives cast by the prose pieces are invaluable.